1. Field of the Invention
The present invention relates to a process for producing pulp which is cooked under alkaline conditions and which is bleached without using chlorine-containing bleaching chemicals, with separate installations being used for recovering and for incinerating/gasifying filtrates from the cooking and the bleaching, respectively.
2. Description of Related Art
Bleaching pulp without using chlorine containing chemicals provides processing solutions which are of great interest from the environmental point of view. Thus, the discharge of impurities into the bleaching plant effluent can be reduced substantially and, in the optimum case, virtually eliminated by, in an appropriate manner, collecting up and concentrating the organic and inorganic compounds in the effluent and subsequently incinerating together the concentrates and the used chemicals from the cooking procedure. A process of this type has been reported in Swedish Patent Application no. 9201477-8.
Bleaching without chlorine-containing chemicals is carried out using hydrogen peroxide, sodium hydroxide and ozone, in particular. These bleaching chemicals are expensive, so that bleaching costs are significantly higher than in the case of conventional bleaching using chlorine and chlorine dioxide. It is therefore desirable, if possible, to be able to recover the bleaching chemicals, especially sodium hydroxide, separately. Sodium hydroxide is principally obtained in association with preparing chlorine gas by the electrolysis of sodium chloride. With decreasing use of chlorine gas, there is some risk of sodium hydroxide becoming an item in short supply.
The process, which is advantageous from the environmental point of view, of returning the used bleaching chemicals, and organic material released during the bleaching, to the system for recovering cooking chemicals can give rise to problems under certain circumstances. If the quantity of sodium compounds and sulphur-containing compounds in the bleaching plant effluent exceeds the requirement for xe2x80x9cmake-upxe2x80x9d chemicals to cover losses, an imbalance arises in the Na/S ratio in the recovery cycle for cooking chemicals. This can lead to problems of having too high an emission of sulphur to the environment, and to other disturbances of the process as well. Another problem, which is equally serious, is that organic material in the bleaching plant effluent, which is incinerated in the works recovery boiler, leads to overloading of the boiler. A works recovery boiler is often utilized to maximum capacity. Overloading results in the production of pulp having to be decreased, which is an economic disadvantage. In those instances where transfer of used bleaching chemicals to the recovery cycle for cooking chemicals can lead to an imbalance in the Na/S ratio and/or overloading of an existing recovery boiler, a separate recovery cycle for the bleaching chemicals would be highly advantageous.
An object of the present invention is to provide a solution, which is technically and economically advantageous, both to the requirement for recovering bleaching chemicals separately and to the problems of limited capacity in the recovery boiler.
In the following, the term bleaching also embraces oxygen delignification.
The invention relates to a process for producing bleached pulp comprising the steps of cooking unbleached pulp under alkaline conditions with a cooking chemical to delignify said unbleached pulp, recovering a filtrate containing a used cooking chemical in a first recovery installation, bleaching said unbleached delignified pulp with a chlorine-free bleaching chemical, and recovering a filtrate containing a used chlorine-free bleaching chemical in a second recovery installation. Preferably, the used cooking and bleaching chemicals are regenerated and reused.